1. Field of the Invention
This invention relates to improvements in utilizing the back EMF generated by a coasting spindle motor. More particularly, this invention uses the back EMF voltage generated to park the head of a driver.
2. Description of the Related Art
Although the invention pertains to polyphase DC motors, in general it is useful with any brushless and sensorless motor. These motors are used to rotate data media, such as hard disk drives, CD ROM drives, floppy disks and other computer related applications.
These motors are typically thought of as having a stator with three coils connected in a "Y" configuration. In actual systems, a large number of stator coils are usually employed with multiple motor poles. In operation, the coils are selectively energized to establish a current path through two coils of the coils forming a "Y" configuration, leaving the third coil to float. Another method sequentially energizes one coil at a time by having a current path through a single coil to a center tap. In either case, a sequence of energized coils is arranged so that as the current paths are changed, or commutated, at least one of the coils used to form the current path becomes a floating coil in the next sequence.
For a three phase DC motor, there are six transistors which control the three stator coils. There are three upper CMOS transistors that connect the coils to a power supply and three lower CMOS transistors that connect the coils to ground. A sequencer generates signals which turn the CMOS transistors on and off in order to change the magnetic field within the stator windings.
These polyphase DC motors are used in hard disk drives. A disk of the disk drive is rotated in a angular direction at a speed provided by the moving rotor (spindle) of the motor. A typical disk has a suitable magnetic material provided on either one (a one-sided disk) or both sides (two-sided disk). The disk is spun by the spindle of the motor. A read-write head, also called the transducer head, engages the magnetic surface of the disk. If the disk has magnetic material on both sides, then two read-write heads are used. A positioner motor moves the read-write head in a radial direction across the disk. The read-write head reads or writes data to the disk. The data is stored on the disk in annular tracks of the magnetic material. The positioner motor typically has a coil (inductor) mounted with a magnetic field. The positioner motor moves the read-write head in the radial direction from points over the disk to a point away from the surface of the disk.
In response to the control signals from the computer, the positioner motor positions the read-write heads over the annular tracks. The disk is spun at a sufficient speed in order to create a film of air which the read-write heads ride on. This prevents the read-write heads from contacting the disk surface which could possibly damage the disk or the read-write heads. The heads are typically designed to actually fly above the disk recording surfaces at heights of less than 50 micro-inches.
If there is a power failure, the disk will begin to slow down and the film of air will subsequently disappear. If the transducer head is still above the disk when this occurs, then irreparable damage can result. Therefore, the read-write head must be withdrawn from the vicinity of the disk before the disk's speed is substantially reduced. In order to "park" the read-write head, the positioner motor must operate when the power supply has failed. Although loss of the power is probably the primary reason for moving the read-write head away from the disk, the procedure is typically also initiated when the disk speed does not remain within tolerance, positioner error is detected, or write circuit faults that could affect stored data are detected.
Upon the detection of a power supply failure, a relay or an equivalent switching means switches a capacitor across a positioner coil terminal to provide the electro-motive force necessary to move the head support structure across the disk surfaces. Near the outer edge of the disk, the read-write head may be supported by a ramp to completely remove the head from the disk.
A rotating magnetic DC motor has kinetic energy stored in the rotating spindle mass. This energy can be used to provide the power required to park the read-write head. In normal operation, the rotating magnetic DC motor is used to drive the spindle. Under a head-parking procedure, the motor stator windings are switched directly to the linear motor positioner coil and the DC motor is converted to a generator to supply the head retract energy. This method utilizes the BEMF generated by the coasting spindle motor. The BEMF is rectified and charges a capacitor which supplies power to the voice coil motor (VCM) in order to "park" the read-write head. Then the driver of the spindle motor control circuitry can short the three phases of the spindle motor in order to "brake" the speed of the disk.
In previous disk drives, the kinetic energy in the stator winding was high enough to run the VCM directly. Due to the progressive size reduction of hard disk drives, the mass of the spindle motors is reduced along with the power supply voltage that is necessary to drive the disk drives. Therefore, the BEMF signal generated by the windings is insufficient in such new disk drives to charge the capacitor. For example, a 3.3 volt application system of the BEMF signal amplitude is generally lower than 3 volts even at full speed (high RPM).